Manufacture of numerous types of thermoplastic olefin polymers now is well known and routinely commercially practiced based on Ziegler-Natta catalyst systems. Useful commercial manufacturing processes for olefin polymers using Ziegler-Natta catalysts have evolved from complex slurry processes using an inert hydrocarbon diluent, to efficient bulk processes using liquid propylene diluent, to even more efficient gas-phase processes in which solid polymer is formed directly from polymerizing gaseous olefin monomer.
Typically-used gas-phase processes include horizontally and vertically stirred sub-fluidized bed reactor systems, fluidized bed systems, as well as multi-zone circulating reactor systems. Thermoplastic olefin polymers made in these processes include polymers of ethylene and C3-C10+ alpha-olefin monomers and include copolymers of two or more of such monomers, such as statistical (random) copolymers or multi-phasic (rubber-modified or impact) copolymers.
Polymers of propylene, which contain crystalline polypropylene segments, are advantageously produced in the gas phase. Such propylene polymers include polypropylene homopolymer in which essentially all of the monomer units are propylene and copolymers of propylene with up to fifty mole percent (50 mole %) of one or more of ethylene or C4+ olefin monomer. Usually, propylene/ethylene copolymers contain up to about 30 wt. %, typically up to about 20 wt. %, of ethylene monomer units. Depending on the desired use, such copolymers may have a random or statistical distribution of ethylene monomer units or may be composed of an intimate mixture of homopolymer and random copolymer chains, typically referred to as rubber-modified or impact copolymers. In such rubber-modified or impact copolymers, typically a high ethylene content random copolymer functions as an elastomeric or rubber component to alter the impact properties of the combined polymer material.
Molecular weight of an olefin polymer, especially propylene polymers, typically is regulated by use of hydrogen in the polymerization gas mixture. A higher concentration of hydrogen will result in a lower molecular weight. The molecular weight distribution of the polymer composition, sometimes referred to as polydispersity, may affect polymer properties.
In horizontal stirred reactors, the average value of the distribution can be controlled by adjusting the inlet Hydrogen flow rate to maintain constant the Hydrogen to Propylene ratio in the off-gas of the reactor. There is a direct link between the average chain length and the gas phase Hydrogen to Propylene ratio. As for the distribution broadness, it cannot be controlled but experience shows that it varies slightly whatever the process conditions of operations. So, some final polymer properties influenced by the broadness of the molecular weight distribution cannot be modified due to inherent reactor limitations. The purpose of the invention is therefore to deal with the broadening of the molecular weight distribution of the polymer made in a horizontal stirred reactor. By applying a hydrogen gradient along the horizontal stirred reactor, the molecular weight distribution can be broadened and controlled in a large range of polydispersity index.
Polymer compositions containing polymer components with different physical properties have been found to have desirable properties. Thus, total polymer compositions containing different amounts of individual polymers in a multimodal distribution may result in a polymer with properties, which are distinct from any of the polymer components. A conventional method of producing multimodal polymers is to blend individual polymers by physical means, such as a blender or blending extruder. A more efficient method of obtaining a multimodal product composition is to produce the product directly in polymerization reactors. In such in situ production, many times a more intimate mixture may be produced, which produces more advantageous properties than are able to be produced by physical blending.
Producing a multimodal product typically requires a process in which polymerization occurs with different conditions at different times or places in the process. Although a single reactor may be used in a batch process to simulate a multi-reactor continuous process, typically batch processes are not practical commercially. A multi-reactor system may be used, which uses two or more reactor vessels.
Gas-phase or vapor-phase olefin polymerization processes are disclosed generally in “Polypropylene Handbook” pp. 293-298, Hanser Publications, NY (1996), and more fully described in “Simplified Gas-Phase Polypropylene Process Technology” presented in Petrochemical Review, March, 1993. These publications are hereby incorporated herein by reference.
A gas-phase reactor system may function as a plug-flow reactor in which a product is not subject to backmixing as it passes through the reactor and that conditions at one part of the reactor may be different from conditions at another so part of the reactor. An example of a backmixed system is a fluidized bed reactor such as described in U.S. Pat. Nos. 4,003,712 and 6,284,848 or a multi-zone system as described in U.S. Pat. No. 6,689,845. An example of a substantially plug-flow system is a horizontal, stirred, subfluized bed system such as described in U.S. Pat. Nos. 3,957,448; 3,965,083; 3,971,768; 3,970,611; 4,129,701; 4,101,289; 4,130,699; 4,287,327; 4,535,134; 4,640,963; 4,921,919, 6,069,212, 6,350,054; and 6,590,131. All of such patents are incorporated by reference herein. Although a single reactor may be used in a batch process to simulate a multi-reactor continuous process in which different conditions are used at different times during a polymerization, typically batch processes are not practical commercially.
The term plug-flow reactor refers to reactors for conducting a continuous fluid flow process without forced mixing at a flow rate such that mixing occurs substantially only transverse to the flow stream. Agitation of the process stream may be desirable, particularly where particulate components are present; if done, agitation will be carried out in a manner such that there is substantially no back-mixing. Perfect plug flow cannot be achieved because the diffusion will always lead to some mixing, the process flow regime being turbulent, not laminar. Since perfect plug flow conditions are not achieved in practice, a plug flow reactor system sometimes is described as operating under substantially plug flow conditions. Ordinarily, plug flow reactors may be disposed horizontally or vertically, and are designed such that they are longer than they are wide (the ratio of the longitudinal dimension to transverse dimension is greater than 1 and preferably greater than 2), the end located at the front of the process stream being referred to as the reactor head or front end, the exit port or take-off being located at the opposite or back end of the reactor.
Depending on manufacturing process conditions, various physical properties of olefin polymers may be controlled. Typical conditions which may be varied include temperature, pressure, residence time, catalyst component concentrations, molecular weight control modifier (such as hydrogen) concentrations, and the like.
In gas-phase olefin polymerization processes, especially propylene polymerization processes, a Ziegler-Natta catalyst system is used composed of a solid titanium-containing catalyst component and an aluminum alkyl co-catalyst component. In propylene polymerizations, which need to control the amount of polypropylene crystallinity, additional modifier components are routinely incorporated into the total catalyst system.
A typical kinetic model used to describe the polymerization reaction rate is a simplified model which assumes a first-order deactivation rate (kd) and first-order dependence of the reaction rate on monomer and active site concentration. Thus,kp=kp0*e(−kd*t) where kp is the polymerization rate (g propylene/h*bar*mg Ti), kp0 is the initial polymerization rate at a time after the process has been lined out (t=0), and kd is the first order deactivation rate.
U.S. Pat. Nos. 3,957,448 and 4,129,701 describe horizontal, stirred-bed, gas-phase olefin polymerization reactors in which catalyst and co-catalyst components may be introduced at different locations along the reactor.
U.S. Pat. No. 6,900,281 describes an olefin polymerization system in which more than one external electron donor is added in a gas-phase polymerization reaction system.
U.S. Pat. No. 5,994,482 describes producing a copolymer alloy in which donor and co-catalyst are added to both liquid pool and gas-phase reactors.
Shimizu, et al., J. Appl. Poly. Sci., Vol. 83, pp. 2669-2679 (2002) describe the influence of alkyl aluminum and alkoxysilane in Ziegler-Natta catalyst deactivation in liquid pool polymerizations.
There is a need for an olefin polymerization process in which product composition may be controlled, especially among different polymerization zones. Also, there is a need for a polymerization process which is able to broaden and control the molecular weight distribution of the polymer made in a stirred horizontal reactor.
This invention comprises a polymerization process that creates a hydrogen gradient within the reactor. Polymers of very different molecular weight are then produced leading to a broadened molecular weight distribution. The homopolymers made in a single reactor under these “hydrogen gradient” conditions have shown better processability and higher melt strength than previously known processes.
This invention has specific application with respect to any types of reactors where the flow pattern is almost a plug flow type, including polymerization of multiple components of the process, including alkyls, electron donors, ethylene and the like.
Thanks to the broadening of the molecular weight distribution, several final properties of the polymer are enhanced without any detrimental effects on other properties. In addition, the process of the invention makes it possible to make